1. Field of the Invention
The present invention relates to the control of communication among a plurality of data processing devices, and more particularly, to a network system which carries out communication according to a token passing style.
2. Description of the Related Art
A token passing style is one of the communication styles that can be employed in a local area network (LAN) which interconnects a plurality of data processing devices.
According to the token passing style, a right-to-transmit information called a xe2x80x9ctokenxe2x80x9d is passed among the processing devices, and each processing device is allowed to transmit data only when possessing the token. The token is passed to the next processing device when a processing device has no data to be transmitted or after lapse of a predetermined hold time or period.
In a ring network, the token is passed to the next processing device along the ring. In a bus network, the token is passed by insertion of the address of the device to which the token is to be passed.
Japanese Patent Application Laid-open No. 7-7504 describes an on-vehicle LAN which interconnects a plurality of electronic controllers mounted on a vehicle. In this on-vehicle LAN, each of the plurality of electronic controllers sends sensor data to a single control-processing-communication unit. On the basis of the received data, the control-processing-communication unit performs arithmetic processing operations and returns a control signal to each of the electronic controllers. A so-called server-client network is established in the on-vehicle LAN.
Japanese Patent Application Laid-open Nos. 5-292102 and 5-316124 describe an on-vehicle LAN which employs a bus to interconnect a plurality of electronic controllers mounted on a vehicle.
Under the conventional token passing style, the token is passed such that each of the plurality of data processing devices constituting the LAN possesses the token for the same amount of time. In a case where there is a great imbalance in the volume of data passed among the plurality of data processing devices, the data processing device which must send a large volume of data requires many token-holding cycles to deliver the large volume of data, which takes a large amount of time and causes a lag in communication.
Accordingly, an object of the present invention is to improve the efficiency of communication among a plurality of data processing devices, which are perhaps unbalanced in terms of the volume of data to be transmitted, in a communications network which passes the right to transmit in the form of a token.
To solve the foregoing problems, according to a first aspect of the present invention, there is provided a network system which includes a plurality of data processors and sequentially passes the right to transmit in the form of a token among the data processors, wherein at least one of the plurality of data processors is set so as to be able, during each possession of the token, to send messages in greater number than those transmitted by each of the remaining data processors by at least one.
By means of the present invention, the volume of message data which can be transmitted during a token holding time can be varied from one data processor to another. Accordingly, efficient communication can be established among a plurality of data processors which are unbalanced in terms of traffic volume.
According to the second aspect of the present invention, the network system as defined in the first aspect is further characterized by a data processor that is allowed to send many messages consecutively, wherein the data processor sends messages by inserting, into a token address field included in a message format, its own address during transmission.
By means of the present invention, the number of messages sent by a data processor can be increased by setting its own address as the token address while the basic configuration of the network is one which sequentially passes the token among a plurality of data processors. Accordingly, efficient communication can be established among a plurality of data processors which are unbalanced in terms of volume of data to be transmitted.
According to a third aspect of the present invention, the network system as defined in the second aspect is further characterized in that the data processors are associated with one another in the form of a server/client relationship, and the data processor that can send many messages is a server.
By means of the present invention, a server can possess the token for a time period substantially longer than those of clients, thereby enabling efficient support of a plurality of clients.